The present invention relates to a die cushion device, e.g., a die cushion device preferably used in deep drawing molding using a press.
In a press working field of a sheet metal, etc., a die cushion device is conventionally arranged below or within a bed of a press machine, and upward reaction force is applied to a lower die by air pressure, etc. so as to preferably perform the press working.
In the main current of a structure of such a die cushion device, the air within a cylinder set to a predetermined pressure is compressed by lowering a piston, and reaction force caused at that time is utilized.
In accordance with Japanese Utility Model Publication No. 24916/1978 and Japanese Utility Model Laid-Open No. 109817/1987, it is proposed that a plurality of such cylinders are arranged in series, and pistons lowered within the respective cylinders are connected to each other, and a supply source of the compressed air and an air chamber within each cylinder compressed by the piston are communicated with each other through an electromagnetic valve.
In the die cushion device described in each of these official gazettes, for example, smallest die cushion ability can be realized by selectively opening and closing the electromagnetic valve if the compressed air is supplied to only one air chamber, and largest die cushion ability can be realized if the compressed air is supplied to all the air chambers. Thus, the die cushionability can be changed by selectively opening and closing the electromagnetic valve
Further, the latter official gazette discloses that the compressed air is also supplied by switching the electromagnetic valve to the air chamber on an upper side increased in volume by lowering the piston, and a pressure receiving face of this piston facing the interior of the air chamber on the upper side is set to have an area smaller than that of a pressure receiving face of this piston facing the air chamber on a lower side.
In such a construction, large die cushion ability can be realized by supplying the compressed air to only the air chamber on the lower side, and small die cushion ability according to the difference in area between the pressure receiving faces can be realized by supplying the air pressure to both the upper and lower air chambers. Therefore, the die cushion ability can be changed even when one cylinder and one piston are used.
In the case of deep drawing molding of a thin plate material, etc., it has been found that it is a useful processing method for improving the quality of molding parts to change a holding degree of the thin plate material in a blank holder by changing the die cushion ability during one stroke as follows.
That is, at an initial stage at which an upper die begins to press the thin plate material, reaction force in the die cushion device is increased so as to firmly hold the thin plate material. At a molding stage from the start of the drawing, the reaction force is reduced so as to preferably perform the drawing. At a final stage at which the molding is terminated, the reaction force is again increased so as to reliably mold an outer shape.
However, in the conventional die cushion device, the die cushion ability is the same as long as the same part is molded if the die cushion ability is once set at a planning time of the press machine. Accordingly, in the conventional die cushion device, a problem exists in that it is impossible to cope with the above-mentioned processing method for changing the die cushion ability during one stroke (during one cycle of the piston of the die cushion device) at a molding time.
Further, the die cushion ability was changed on trial during one stroke by switching the opening and closing of the electromagnetic valve of the conventional die cushion device during one stroke. However, a problem exists in that responsiveness of the electromagnetic valve is bad and no die cushion ability can be smoothly changed.